Cathode ray tube (CRT) displays have been available for many years. The first cathode ray tube applications were primarily in television and oscilloscopes. Later, as computers developed, information display devices, employing a CRT responsive to a general purpose or special purpose digital computer, were developed to provide a two level, black-white, alphanumeric display. Often, the video amplifier driving the computer responsive display was the same video amplifier used in analog (television and oscilloscope) applications and consequently the earlier computer responsive displays had severe bandwidth constraints compared with the available digital switching rates. Soon thereafter, however, CRT displays for presenting alphanumerics employed video amplifiers having on-off solid state switching so that one brightness level and a black level could be displayed on the cathode ray tube face at a relatively high bandwidth.
As the applications in which the cathode ray tube displays were used increased in both complexity and sophistication, and to increase their appeal for the user, two levels of brightness (plus a black level) were provided to allow the user the option of emphasizing selected display material by using a different brightness background.
As the display applications of cathode ray tubes became yet more sophisticated, there continued a pressing need to increase the speed (or bandwidth) of the video circuitry driving the tube to provide a faster display and hence a "crisper" display, that is, a display having sharper edges and hence a generally more pleasing aesthetic appearance at high display rates. As a result, the assignee of the present invention has provided video displays having two brightness levels plus a black level (a total of three video levels) which operate at a video bandwidth of approximately 25-50 MHz.
A principle object of this invention is a video amplifier capable of operating at still higher speeds while providing greater brightness level flexibility. Other objects of the invention are a video amplifier having a relatively low cost, high reliability, and which can be a plug-in replacement for present slower video amplifiers.
Still further objects of the invention are a video amplifier having a flexible decoding arrangement wherein different digital input signal coding can be accommodated.